In drilling a borehole in the earth, such as for the recovery of hydrocarbons or for other applications, it is conventional practice to connect a drill bit on the lower end of an assembly of drill pipe sections which are connected end-to-end so as to form a “drill string.” FIG. 1 includes a drilling installation having a drilling rig 10 at the surface 12 of a well, supporting a drill string 14. The drill string includes a bottom hole assembly 26 (commonly referred to as a “BHA”) coupled to the lower end of the drill string 14. The BHA includes the drill bit 32, which rotates to drill the borehole. As the drill bit 32 operates, drilling fluid or mud is pumped from a mud pit 34 at the surface into the drill pipe 24 and to the drill bit 32. After flowing through the drill bit 32, the drilling mud rises back to the surface via an annulus between the outside of the drill pipe and the borehall wall, forming a mud column. The drilling fluid that travels all the way to the surface is collected and returned to the mud pit 34 for filtering. In drilling a well, due to the hydraulic pressure of the mud column in the borehole, the formation zone that is immediately behind the borehole wall is “flushed”, which means that formation fluid contained in the rock pores is displaced to some degree by the mud filtrate. The mud, in the process of filtering into the formation, also tends to leave a mudcake on the wall of the hole.
Modern oil field operations demand a great quantity of information relating to the parameters and conditions encountered downhole. Such information typically includes characteristics of the earth formations traversed by the wellbore, in addition to data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as “logging,” can be performed by several methods.
Logging has been known in the industry for many years as a technique for providing information regarding the particular earth formation being drilled or that has been drilled. In conventional oil well wireline logging, a probe or “sonde” is lowered into the borehole after some or all of the well has been drilled, and is used to determine certain characteristics of the formations traversed by the borehole. The sonde may include one or more sensors to measure parameters downhole and typically is constructed as a hermetically sealed steel cylinder for housing the sensors, which hangs at the end of a long cable or “wireline.” The cable or wireline provides mechanical support to the sonde and also provides an electrical connection between the sensors and associated instrumentation within the sonde, and electrical equipment located at the surface of the well. Normally, the cable supplies operating power to the sonde and is used as an electrical conductor to transmit information signals from the sonde to the surface, and control signals from the surface to the sonde. In accordance with conventional techniques, various parameters of the earth's formations are measured and correlated with the position of the sonde in the borehole, as the sonde is pulled uphole.
Designs for measuring conditions downhole and the movement and the location of the drilling assembly, contemporaneously with the drilling of the well, have come to be known as “measurement-while-drilling” techniques, or “MWD.” Similar techniques, concentrating more on the measurement of formation parameters of the type associated with wireline tools, commonly have been referred to as “logging while drilling” techniques, or “LWD.” While distinctions between MWD and LWD may exist, the terms MWD and LWD often are used interchangeably. For the purposes of this disclosure, the term LWD will be used generically with the understanding that the term encompasses systems that collect formation parameter information either alone or in combination with the collection of information relating to the position of the drilling assembly.
Ordinarily, a well is drilled vertically for at least a portion of its final depth. The layers, strata, or “beds” that make up the earth's crust are generally substantially horizontal, such as those labeled 20, 21, and 22 in FIG. 1. Therefore, during vertical drilling, the well is substantially perpendicular to the geological formations through which it passes. A sudden measured change in resistivity by a resistivity tool generally indicates the presence of a bed boundary between layers. For example, in a so-called “shaley” formation with no hydrocarbons, the shaley formation has a very low resistivity. In contrast, a bed of oil-saturated sandstone is likely to have a much higher resistivity.
A number of regions can be defined in and around the borehole. Referring to FIG. 11, the area inside the wellbore contains drilling mud and has a resistivity of Rm. The mudcake has a resistivity of Rc. The region of the formation behind the borehole wall invaded by drilling fluid, also referred to as the flushed zone, has a resistivity of Rxo.
The resistivity of the flushed zone (Rxo), is of petrophysical importance. For example, the resistivity of the flushed zone is useful in estimating the movability of formation hydrocarbon. Therefore, an accurate Rxo value with a reasonably large dynamic range is desirable for successful well log interpretations. A device for measuring flushed zone resistivity should measure only a very shallow depth immediately behind the borehole wall (to ensure measurement at Rxo). It also should be immune to borehole rugosity or mudcake effect.
Currently, the predominant Rxo device for use in conductive mud is referred to as the Micro Spherically Focused Log (MSFL). It is a tool that provides a shallow measurement into formation behind the borehole wall. In general, an MSFL device gives a reliable Rxo reading when the resistivity ratio between Rxo and mud resistivity, Rm, is not very high, e.g. less than 10,000. But if this ratio is high, e.g. greater than 10,000, the tool becomes sensitive to the presence of mudcake or standoff effect. Under this situation, the Rxo measurement from an MSFL device may have 100% or even larger error. This has made MSFL measurement in highly resistive formation less reliable. A more accurate resistivity tool is needed, especially where this ratio is high. It would also be desirable if more accurate measurements of other formation and borehole properties could also be obtained.